Electron paramagnetic resonance

Electron Paramagnetic Resonance

Electron paramagnetic resonance

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Description:
Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion. The basic physical concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of spin of atomic nuclei. Because most stable molecules have all their electrons paired, the EPR technique is less widely used than NMR. However, this limitation to paramagnetic species also means that the EPR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to EPR spectra.

EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at the University of Oxford.



Theory

Origin of an EPR signal

Every electron has a magnetic moment and spin quantum number s = 1/2, with magnetic components m<sub>s</sub> = +1/2 and m<sub>s</sub> = −1/2. In the presence of an external magnetic field with strength B<sub>0</sub>, the electron's magnetic moment aligns itself either parallel (m<sub>s</sub> = −1/2) or antiparallel (m<sub>s</sub> = +1/2) to the field, each alignment having a specific energy (see the Zeeman effect). The parallel alignment corresponds to the...
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